464 Geology of the South-East of Dorsetshire. 



tion a to b, till the stratum a c be brought into the position 

 indicated by the dotted line a c; and the same arrangement 

 of strata which actually takes place in the cliff will be the 

 result. In order to produce this agreement, we have only to 

 suppose that the line of fault, during a part of its course, co- 

 incides with the lines of stratification, which cannot be con- 

 sidered as an impossible, or even improbable, circumstance." 



The writer then proposes to copy " the curved and vertical 

 strata, as represented in Mr. Webster's delineation, on two 

 separate species of paper ; then filling up the vertical strata, 

 as represented in the above diagram; and, lastly, applying 

 them together and slipping the latter under the former, in the 

 direction of the fault ; " he will perceive that " this simple 

 supposition is completely adequate to the solution of the actual 

 phenomena." (Outlines, p. Ill, 112.) 



In the above explanation, there is an assumption that the 

 strata were originally in some such predicament as Mr. Web- 

 ster supposes, forming part of a huge dome or arch, which once 

 extended 3000 or 4000 ft. higher than any portion of the 

 present elevation of Purbeck, and which has been denuded 

 altogether, leaving only a fragment of its northern side. 



It appears to me that a third solution may be offered, with 

 more chance of success ; and, accordingly, I venture to sug- 

 gest some such explanation as the following : — 



The line of fault is, no doubt, accurately assumed ; its 

 presence is actually visible upon close inspection : more- 

 over, it forms the north side of the Purbeck Hills, as may be 

 seen on the outside of Challow Hill, a little to the east of 

 Corfe Castle, where the beds towards Poole are nearly 

 vertical, the flints being fractured, and dispersed; the par- 

 ticles separated by chalk and carbonate of lime in crystals ; 

 and the chalk itself as hard as marble, having no streak, and 

 being only reducible to the soft state by scraping. 



These phenomena characterise the chalk at the end of the 

 range, under Ballard Down, and are traceable all through 

 the elevated ridge, in the steep slope towards the north. 



Now, in the hypothesis that the chalk was formerly alto- 

 gether horizontal through Purbeck, and that elevation began 

 in the direction of the supposed fault (i. e. by forcing the beds 

 of the whole mass now called Purbeck, to the south, at the 

 same time that they were raised vertically), the chalk 

 would naturally suffer from the strain ; but, as it probably did 

 not extend far to the south at the immediate period of the 

 fault's formation, and the northern extremity of the chalk was 

 weighed down by the superincumbent burden of the tertiary 

 beds, which, near Poole, have a thickness of more than 



